A fixing device of a heat roller fixing method is widely used as a fixing device to be provided in an image forming apparatus of an electrophotographic printing method such as a copying machine or a printer. In general, a fixing device of a heat roller fixing method includes a pair of rollers (fixing roller and pressure roller) which are pressed against each other. The pair of rollers is heated to a predetermined temperature (fixing temperature) by heating means which is made up of a halogen heater etc. provided inside each or either one of the pair of rollers. Then, a sheet of recording paper on which an unfixed toner image is formed is fed to a pressure area (fixing nip area) of the pair of rollers so that the sheet of recording paper is passed through the pressure area. Thus, the toner image is fixed onto the sheet of recording paper by application of heat and pressure.
In a fixing device to be provided in a color image forming apparatus, in general, an elastic roller is adopted as the fixing roller. The elastic roller has, as its surface layer, an elastic layer made of silicone rubber or the like. By adopting the elastic roller as the fixing roller, the surface of the fixing roller undergoes elastic deformation in accordance with asperities of an unfixed toner image. Accordingly, the surface of the fixing roller makes contact with the unfixed toner image so as to wrap the unfixed toner image. This makes it possible to properly fix, onto a sheet of recording paper, an unfixed color toner image which is larger in its toner amount than that of a monochrome one. In addition, an effect of releasing deformation of the elastic layer in the fixing nip area allows the fixing roller to have improved releasability to color toner which is more likely to undergo offset than a monochrome one. Furthermore, the elastic layer of the fixing roller deforms so as to have a concave shape. Accordingly, the fixing nip area has a nip shape which is convex upward (i.e., convex toward the fixing roller; a so-called inverse nip shape). This makes it possible to improve an ability of the fixing roller to peel a sheet of recording paper from the fixing roller. Accordingly, the sheet of recording paper can be peeled from the fixing roller, without the use of peeling means such as a peeling claw (self-stripping). This makes it possible to eliminate a defect of an image caused by the peeling means.
However, in some cases, the fixing device to be provided in the color image forming apparatus cannot properly fix an unfixed toner image onto a sheet of recording paper at a high processing speed (passing speed of the sheet of recording paper in the fixing nip area; fixing speed) because sufficient heat cannot be applied to the unfixed toner image. Therefore, it is necessary to widen a nip width of the fixing nip area in order to properly perform a fixing process at a high processing speed by applying sufficient heat to the unfixed toner image. Possible methods for widening the nip width are (1) increasing a thickness of the elastic layer of the fixing roller and (2) increasing a diameter of the fixing roller.
However, the elastic layer has an extremely low heat conductivity. Therefore, an arrangement in which, as is the case with a conventional one, the fixing roller has the elastic layer and contains the heating means has a problem in that a surface temperature becomes less likely to follow a temperature change of the heating means in a case where the processing speed is increased. Accordingly, the problem of the weakening follow-up behavior of the surface temperature of the fixing roller becomes prominent in a case where the elastic layer is thicken by the method of (1) above. Furthermore, the method of (1) has a problem of a longer warm-up time for heating and a problem of increased power consumption, because of the low heat conductivity of the elastic layer.
On the other hand, in a case where the diameter of the fixing roller is increased by the method of (2) above, there arise a problem of a longer warm-up time for heating and a problem of increased power consumption, because the fixing roller has a large heat capacity due to its increased size.
As a solution to the problems, an art (external heating and fixing method) is proposed in which external heating means is pressed against the surface of the fixing roller so that the fixing roller is externally heated.
For example, Patent Literature 1 discloses a fixing device of an external belt heating and fixing method. The fixing device includes: a fixing member; an endless belt (external heating belt) suspended by a plurality of suspension rollers; and heating means for heating the endless belt. The fixing device heats the fixing member by pressing the endless belt against the fixing member.
The fixing device which adopts the external belt heating and fixing method externally heats the fixing roller by use of a belt having a small heat capacity. This makes it possible to heat the fixing roller swiftly, and thereby reduce a warm-up time. Therefore, it is possible to secure a wide nip width by providing a thick elastic layer having a low hardness to the fixing roller, and/or increasing a diameter of the fixing roller, while preventing problems such as the weakening follow-up behavior of the surface temperature of the fixing roller and the increase in warm-up time.
Patent Literature 1
Japanese Patent Application Publication, Tokukai, 2007-212896 A (Publication Date: Aug. 23, 2007)